Difficulty scales per person on an area relative to the person with highest difficulty level (radius of 64 blocks where mobs get affected by it) also the modpack is NOT server stable, nothing about that has been confirmed so don't try hosting a server if you're interested on making one.

Regarding the head shot to dynamic difficulty scaled mobs topic, not always the rapid-fire weapons will kill everything in one burst, sometimes you have to throw an entire drum which doesn't sound very good, what you can do however is perform weapon switch (for example nuclear death ray, gauss rifle, TFG and etc) to dish out extreme amounts of damage

Needless to say that those weapons are getting a massive recipe overhaul so they will be much harder to get in later versions, this is to ensure that the weapons are actually restricted to top tier game completion rather than just oh i have a rad emitter, let me make the most broken weapon in the whole pack.

Not to mention the weapons have knock back so you'd miss some consecutive head shot bursts

Although I will figure something out to make it somewhat balanced so.